Previous studies have demonstrated that lithium ion exerts a strong stimulatory effect on cell multiplication of mouse mammary gland in culture. This work aims at establishing whether lithium ion: a) can increase the tumor inducing capability of small doses of NMU, that usually are tumorigenic only in 5-10 percent of the treated rats and b) can modify the curve of appearance of tumors in DMBA-treated rats that have not developed carcinomas 4 months after treatment.